The present invention relates to an improvement of a progressive multi-focal ophthalmic lens which is characterized by a continuous change in refractive index from the portion of far distance vision to that of near distance vision.
Progressive multi-focal ophthalmic lenses are those lenses which are used in spectacles for the aged and in which there is no boundary between the portion of far distance vision and that of near distance vision. Various proposals have been made with respect to progressive multi-focal ophthalmic lenses as shown in JP-A-57-139718 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-88718, JP-A-60-260917 and JP-A-61-252526.
Three examples of the prior art progressive multi-focal ophthalmic lenses are described below together with specific numerical data, and with the problems associated with these examples being pointed out as the need arises.
Lens 1 to be described below has a basic geometry as shown in FIG. 1; it has a convex front refracting surface 1a and a concave rear refracting surface 1b, with the front refracting surface 1a being a progressive surface whose refractive power changes continuously and the rear refracting surface 1b being a spherical surface having a curvature radius of R.sub.2.
Also assume here an x-axis which, as shown in FIG. 1, passes through the center of the lens 1, as well as y- and z-axes that are perpendicular to the x-axis and which cross each other at right angles in a plane tangent to the front refracting surface 1a of the lens 1.
It is generally known that a smooth three-dimensional curved surface as expressed by f(x,y,z)=0 is reasonably approximated within a certain region by the following two-variable high order polynomial: ##EQU1## The front refracting surface 1a shown in FIG. 1 is an aspherical surface as expressed by a two-variable 16th order polynominal having the asphericity coefficient. An the specific values of which are shown in Tables 1-1 and 1-2 to be referenced later in this specification. This aspherical surface has a shape as shown -in FIG. 2 by way of a contour line diagram (the height distance between solid lines is 1 mm, which applies to the description which follows), a horizontal cross section and a vertical cross section. The notation "E" in Table 1 means "exponent" and "-8.590E-18", for example, means "-8.590.times.10.sup.-18 ". The same applies to the other tables given in this specification.
The three examples of the prior art progressive multi-focal ophthalmic lens which are described below are the same in that the front refracting surface 1a has the shape defined above and that the outside diameter of the lens is 70 mm, with the constituent material being an acrylic resin (n=1.492). However, these lenses differ in refractive power because of the differences in the shape of the rear refracting surface 1b.